1. Field of the Invention
The present invention relates to an automatic method of axial adjustments for use in an electron beam system, such as an electron microscope equipped with an aberration corrector.
2. Description of Related Art
One of the most important factors in deriving an image by a scanning electron microscope is resolution. Resolution is an index indicating the distance between two points that are discernible within a space or object. Improvement of resolution is the eternal theme in electron microscopy, as well as in scanning electron microscopy. One method of improving resolution is to shorten the distance (working distance) between the objective lens and the specimen. Another method consists of reducing various aberrations.
The aberrations include diffraction aberration, spherical aberration, and chromatic aberration. Of these aberrations, the diffraction aberration can be reduced by increasing the angular aperture. Furthermore, the spherical aberration and chromatic aberration can be reduced by improvements of electron optics. In recent years, various methods have been developed.
For example, a technique of reducing or correcting spherical and chromatic aberrations by incorporating an aberration corrector into the electron optical system has been proposed and is almost put into practical use. A proposed method for use in such an aberration corrector consists of correcting chromatic aberration by means of an electrostatic quadrupole lens and a magnetic quadrupole lens and correcting spherical aberration by means of four stages of electrostatic octopole lenses. The principle of this correction of aberrations is described in detail, for example, by H. Rose in Optik 33, Heft 1, 1-24 (1971) and by J. Zach in Optic 83, No. 1, 30-40 (1989).
Another known aberration corrector is made up of four stages of electrostatic quadrupole lenses, two stages of magnetic quadrupole lenses for superimposing a magnetic potential distribution analogous to the electric potential distribution created by the central two stages of the four stages of electrostatic quadrupole lenses, and four stages of electrostatic octopole lenses for superimposing an octopole electric potential on the electric potential distribution created by the four stages of electrostatic quadrupole lenses (for example, U.S. Pat. No. 6,852,983, paragraphs 0023-0027 and FIG. 3).
One example of aberration corrector is now described briefly. FIG. 1 shows a part of the electron optical system of an electron beam system equipped with an aberration corrector.
For example, the aberration corrector, indicated by 40, is made up of four stages of electrostatic quadrupole lenses 1, 2, 3, 4, two stages of magnetic quadrupole lenses 5, 6 for superimposing a magnetic potential distribution analogous to the electric potential distribution created by the two central stages of electrostatic quadrupole lenses 2, 3 out of the four stages of electrostatic quadrupole lenses, four stages of electrostatic octopole lenses 7, 8, 9, 10 for superimposing an octopole electric potential on the electric potential distribution created by the four stages of electrostatic quadrupole lenses, and four stages of electrostatic dipole lenses 11, 12, 13, 14 for superimposing a dipole electric potential on the electric potential distribution created by the four stages of electrostatic quadrupole lenses. The electron beam transmitted through the aberration corrector 40 is focused onto a specimen 16 by an objective lens 15.
Reference orbits extending in the X- and Y-directions, respectively, are indicated by Rx and Ry, and are paraxial orbits that are assumed where there is no aberration. The quadrupole lens 1 causes the Y-direction reference orbit Ry to pass through the center of the quadrupole lens 2. The quadrupole lens 2 causes the X-direction reference orbit Rx to pass through the center of the quadrupole lens 3. Finally, the quadrupole lenses 3, 4 and objective lens 15 cause the orbit of the electron beam to be focused onto the specimen 16. Spherical and chromatic aberrations are corrected by this aberration corrector as follows.
With respect to correction of the chromatic aberration, chromatic aberration in the X-direction in the whole lens system is corrected to zero by adjusting the electric potential V2 at the electrostatic quadrupole lens 2 and the excitation B2 at the magnetic quadrupole lens 5 in such a way that the reference orbits are kept unchanged. Similarly, chromatic aberration in the Y-direction in the whole lens system is corrected to zero by adjusting the electric potential V3 at the electrostatic quadrupole lens 3 and the excitation B3 at the magnetic quadrupole lens 6 in such a way that the reference orbits are kept unchanged.
The spherical aberration is corrected after the correction of the chromatic aberration. In particular, the spherical aberration in the X-direction in the whole lens system is corrected to zero by the electric potential V2 at the electrostatic octopole lens 8. The spherical aberration in the Y-direction is corrected to zero by the electric potential V3 at the electrostatic octopole lens 9. Spherical aberration in the combined direction of the X- and Y-directions is corrected to zero by the electrostatic octopole lenses 7 and 10. The accuracy can be improved by repeatedly carrying out the above-described operations.
In order to focus the electron beam onto the specimen 16 reliably, mutual adjustments of the reference orbits in the X- and Y-directions are necessary. For this purpose, axial adjustments of the whole aberration corrector are necessary. However, such axial adjustments are made differently by each different operator.
Furthermore, such axial adjustments are not being constantly made. Consequently, a considerably long time is taken until a high-resolution image is obtained. In addition, axial adjustments of such an aberration corrector are complex to perform. Often, the axial adjustments are made unsuccessfully.